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Author Topic: Mobile 2 Meter 1/4 Wave Whips Have High SWR but 5/8 Checks Fine?  (Read 9542 times)
W4JST
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« on: April 16, 2012, 06:02:10 PM »

I bought a couple 19" or whatever 2 meter 1/4" stainless steel whips with 3/8" threads.

I have a hole drilled in my roof with a 3/8" permanent mount.

My analyzer shows the 1/4 wave whip SWR to be well over 2:1 maybe 3:1 and up not just all over two meters, but everywhere.

I stuck my 5/8 wave on the same mount and the SWR is 1.6:1 or so.

I thought maybe it was all the metal of this large van roof so I tired it on the SUV, same type of permanent mount, and the same thing, none of my 1/4 wave whips check right but the 5/8 wave SWR was fine.

Can anyone tell me what's going on?
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KD0PBO
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« Reply #1 on: April 16, 2012, 08:12:05 PM »

Maybe a short in the antenna somewhere?? Does it have a loading coil of some type or is it a straight steel whip?

Only thing I can think of to cause high SWR regardless of frequency..

KD0PBO
Miles D.
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W4JST
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« Reply #2 on: April 17, 2012, 05:30:56 PM »

No, this is just a 1/4 wave two meter whip. I had two. Nothing but a stainless steel whip with 3/8" fine threads at the base.

I even made one. I took a 3/8" fine thread bolt and heated it with the torch and soldered a piece of about 19" copper wire to it and it checked the same way.

Maybe a 1/4 wave two meter whip can't have too big of a ground plane or is funny with certain mounts or something? The 5/8 wave checks fine.

Thanks,
amduke@mail.com
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KCJ9091
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« Reply #3 on: April 17, 2012, 07:46:22 PM »

What mount are you using?
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G7MRV
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« Reply #4 on: April 18, 2012, 04:01:12 AM »

How are you checking the SWR? You say its high 'everywhere', you say your using an analyser, is the analyser working ok?

Ive never known a straight 1/4w give problems, but you will need to trim it a little, just using a ballpark 19" isnt likely to be spot on. Have you trimmed it? It may be that you were low to start with and have trimmed it even further out, have you checked higher and lower than 2m? I would start with a whip a couple inch longer and trim it down for 2m, rather than starting where the formula says, that way you should trim it into the band with less risk of trimming it out of the band!

Are you sure that the mount is providing a good ground connection? Did you check continuity between the car bodywork and the coax (without the coax connected to a radio!)?

You did mention trying them on another vehicle, this suggest to me its the whips rather than the mount.
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KA3NXN
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« Reply #5 on: April 18, 2012, 07:58:22 AM »

If your analyzer is working properly and the installation of the antenna is correct, than tuning around with your analyzer the SWR should drop. if it drops at a higher frequency than 2 meters than the antenna is too short, if it drops a a lower frequency than you 2m frequency it's too long. One way or another it should drop if the antenna is installed properly and your analyzer is working properly. I had an analyzer that drove me crazy because I couldn't get my swr down below 2.8:1 and then I put a dummy load on the analyzer and guess what? 2.8:1. BAD ANALYZER!!! Drove me crazy thinking that the problem was in my antenna. Also, is there anything near the whip like a truck box or anything metal that could possibly be detuning the 1/4 wave whip? That will reek havoc on an installation.


Jaime-KA3NXN
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W4JST
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« Reply #6 on: April 18, 2012, 09:03:44 AM »

Hi, thanks for the ideas everyone.

This is a MFJ-269 analyzer. I bought it new.

It works fine for checking CB antennas, HF dipole, 1/4 wave 2 meter magnetic mount, 5/8 wave magnetic mount, as well as a 5/8 wave whip on the exact same mount I checked three different 1/4 waves on.

The mount is in the open, on top of a Sprinter van roof, but near the back. About 6 feet behind a 4' fiberglass CB antenna. My 5/8 wave works fine in this spot.

Yes, I tuned below and above the two meter band and the SWR varies but is not right anywhere. The ground etc. is fine. I use my HF antenna and 5/8 wave antenna on this same mount, but have not gotten a 1/4 wave to work on it. I've tried three different ones.

I also tried a couple of the 1/4 waves on the roof of an SUV. Same type of 3/8" fine thread mount like CB antennas use, the SWR was high all around two meters. My 5/8 wave whip checked fine.
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W4JST
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« Reply #7 on: April 18, 2012, 09:23:27 AM »

I just took my WaveNode out there and checked the three 1/4 wave whips with it, across the two meter band. The best I saw was 2.0:1. That was at 144 MHz with the longest whip. I then tried the two shorter whips and they weren't any better. I don't think I saw it below 2:1 anywhere on the analyzer.

I don't think the analyzer is bad, but not sure why I can't get below 2:1 anywhere near the 2 meter band on a 1/4 wave whip. Would too much ground plane (too large of a vehicle) cause this? I wouldn't have though it would've been a problem on the SUV but it was a thought on the van.
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KCJ9091
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« Reply #8 on: April 18, 2012, 01:17:58 PM »

How are your sure the ground is fine?  Did you check the outer conductor of the connector to the metal body of the vehicle  with the cable disconnected from the radio?
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G7MRV
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« Reply #9 on: April 18, 2012, 01:31:31 PM »

No, you cant really have too much groundplane. You can most certainly have too little, but not too much!

Remember, at resonance a quarter wave on a car roof should present an impedence of about 30ohm, and so you should be aiming for a VSWR of 1.5:1, and no lower. 2:1 should be well within your radios tolerances, but may cause a bit of ramp-back of the power level.

How far outside the band did you run the analyser? At VHF its worth going quite a way out to build up a picture. Your 2m band extends further than ours in the UK, but i would suggest checking to a good few MHz beyond the band limits.

I suspect you will find it to be resonant somewhere a little outside the band.

I dont know your type of analyser, I use an MFJ-259B, and i watch for the X reading to be as close to 0 as possible, indicating very low reactance, and hence resonance, rather than watching the SWR reading.
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W4JST
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« Reply #10 on: April 18, 2012, 02:28:07 PM »

I just took one of my 1/4 wave whips, the one made out of copper wire soldered to a 3/8 fine thread bolt and watched for X to be about 0 and for the SWR to be lowest and it was around 120 MHz so I trimmed a little bit at a time until the lowest SWR and X=0 worked it's way up into the 2 meter band and the best I got was about 2.3:1 in the 2 meter band.

I took the same whip, put it on the SUV and checked and it's lowest SWR with X=0 was around 154 MHz and was about 1.7:1.

Indicating the antenna was too short, I took the two longer, stainless steel store bought 1/4 wave whips and tried each of those and the SWR was not below about 2:1 anywhere.

Three different antennas, two different cars/mounts, greater than 2:1 SWR across the 2 meter band no matter how they're trimmed.

I'd like to know more about this. I was thinking the feedpoint of a halfwave dipole was around 50 ohms and a 1/4 wave on a car was about the same thing. Why is it 30 ohms? There's no length that would present closer to 50 ohms for a mobile installation?

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W4JST
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« Reply #11 on: April 18, 2012, 02:40:11 PM »

KCJ9091 and the group,

I just checked continuity from the PL-259 shield and the car body as well as from the PL-259 center pin and the antenna mount on BOTH cars and did not see any resistance in either case.

On both vehicles, the antenna mount is around 6" from the back of the roof, would an offset ground plan or anything like that keep me from achieving below 2:1 SWR regardless of frequency?

Thanks everyone.
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G7MRV
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« Reply #12 on: April 18, 2012, 03:34:49 PM »

OK. Sounds like something very odd is going on. The position of the mount on the groundplane shouldnt have any effect on the match, though it may affect the radiation pattern. It seems very odd that the same whip measured long on one vehicle but short on the other. Can you measure the length of that whip after trimming and post the result (ideally for me in mm but use inches if you like, i'll convert it)

If I were you, I would take the longest whip (or ideally a new whip of about 24") and work solely on that one antenna until you get somewhere, that way you cant mix up results from the different whips. Test the connections of the mount, between the bodywork and from the center. Check the resistance of the coax outer AND the inner between the mount and coax connector, waggle it about to rule out a dry joint or other poor connection. Check between the pin and outer while wiggling it about also to rule out any intermittent shorts.

Then, with the longest whip installed, measure the resonant point, and then trim slowly, 1/4-1/2 an inch at a time, until the resonant point is close to where you want it, trim more slowly once nearly there so you dont overshoot.

Once you have that whip where you want it, then try it on the other vehicle.

Post up the results you find on here.

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KCJ9091
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« Reply #13 on: April 18, 2012, 07:27:24 PM »

The antenna mount is not giving you a useable ground if you do not have a reading of 5 ohms or less from the connector shield to body.  A quarter wave will give you all kinds of strange readings if the shield is not making contact at the antenna end.  Are you sure the roof of the Sprinter is metal?
Is there paint keeping the ground part of the mount from making good contact if it is metal?
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W4JST
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« Reply #14 on: April 19, 2012, 06:40:29 AM »

G7MRV,

That's what I did. I started with one whip, it was resonant (at least 2:1 or something and X=0) around 120 MHz. I slowly trimmed until the "resonant" spot worked it's way up to 146 MHz but the lowest SWR at the point even was 2.3:1

KCJ9091,

The resistance was 5 ohms or less. I measured continuity. Both mounts check out fine. The roof is definitely metal and I have a good ground connection.


Thanks everyone.
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